This invention relates to an exposure control method for controlling the amount of exposure of a photosensitive substrate in an exposure apparatus used in a lithographic process for manufacturing, e.g., a semiconductor element, a liquid crystal element, an image sensing device (CCD, etc.) or a thin-film magnetic head, as well as to an exposure apparatus and device manufacturing method capable of employing this control method. The present invention is applicable not only to a batch exposure type exposure apparatus but also in a case where the amount of exposure is controlled in a step-and-scan scanning exposure type projection exposure apparatus in which part of the pattern on a mask (reticle) is projected onto a photosensitive substrate and the mask and substrate are then scanned synchronously with respect to a projection opticals unit, whereby the mask pattern is transferred to shot areas on the substrate to expose these areas to the mask pattern.
FIG. 2 illustrates a projection exposure apparatus according to the prior art. The apparatus includes a light source 1 such as a high-voltage mercury-vapor lamp which emits illuminating light. The light from the light source 1 is condensed to a point by a condensing mirror 2 and impinges upon a fly-eye lens 4 through an opticals unit 3. There are instances where a laser or the like may be used as the source of illuminating light, in which case the condensing mirror 2 is unnecessary and the light from the laser need only impinge upon the fly-eye lens 4 through the opticals unit 3.
The fly-eye lens 4 is a bundle of rod lenses the entrance and exit surfaces of which have their focal points on each other""s surface. A group of light beams that impinge upon the rod lenses at an identical angle are condensed at the exit surfaces and form a number of points of condensed light on the exit surface of the fly-eye lens.
Utilizing the group of condensed points of light formed on the exit surface of the fly-eye lens 4, the opticals unit 5 uniformly illuminates the position of a diaphragm 6, which controls an illuminated area at a position that is conjugate with the plane of a mask 8. An opticals unit 7 is for forming the image of the position of the uniformly illuminated diaphragm 6 on the mask surface 8. Uniform illumination of the mask surface 8 is achieved by forming the image of the position of the uniformly illuminated diaphragm 6 on the mask surface 8. It should be noted that the position of mask 8, the position of diaphragm 6 and the entrance surface of the fly-eye lens 4 are located at conjugate points.
The apparatus further includes a projection opticals unit 9 for forming the image of the pattern of mask 8 on a substrate 11. A photosensitive agent that has been applied to the substrate 11 is exposed to the mask pattern by the illuminating light from the illuminating opticals unit. The projection opticals unit 9 is a telecentric unit in which projection magnification does not change even if the position of the mask 8 or the position of the substrate 11 shifts along the optical axis. The arrangement is such that a principal ray which passes through the center of the projection unit at the position of a diaphragm 10 perpendicularly intersects the surface of the mask 8 and the substrate 11.
It should be noted that the diaphragm 10 of the projection opticals unit 9 and the exit surface of the fly-eye lens 4 are located at conjugate points.
The apparatus further includes a movable stage 12 on which the substrate 11 and an exposure sensor 15 are mounted. The exposure sensor 15 can be moved over the illuminated area when the amount of exposure at a position identical with that of the substrate 11 is measured with stepping movement for exposing a plurality of shots on the substrate 11.
In such a projection exposure apparatus used in the manufacture of semiconductor devices and the like, it is required that the substrate be subjected to a proper amount of exposure, which depends upon the photosensitive agent that has been applied to the substrate and the pattern possessed by the mask 8, in order that the mask pattern will be transferred to the substrate in optimum fashion. If the amount of exposure is less than the proper amount in a case where a positive pattern and a negative resist are used, for example, the photosensitive agent will not be sensitized sufficiently and the lines of the pattern may become too fine and be rendered discontinuous at points. If the amount of exposure is too large, on the other hand, the photosensitive agent will be sensitized excessively and the lines of the pattern may become so thick that neighboring lines will contact each other. Further, if the amount of exposure is less than the proper amount in a case where a negative pattern and a positive resist are used, the photosensitive agent will not be sensitized sufficiently and the lines of the pattern may become so thick that neighboring lines will contact each other. If the amount of exposure is too large, on the other hand, the photosensitive agent will be sensitized excessively and the lines of the pattern may become too fine and be rendered discontinuous at points. In any case, when exposure is carried out with an improper amount of exposure, a suitable pattern cannot be formed on the substrate. This invites a decline in yield when semiconductor devices or the like are manufactured.
Control of the amount of exposure to which the substrate 11 is subjected must be controlled in order to obtain the proper amount of exposure. However, the amount of exposure being applied to the substrate 11 cannot be measured directly during the transfer of the pattern of mask 8 to the substrate 11. If the amount of exposure is measured along the optical path of the exposing light, the shadow of the exposure sensor has an influence when the mask pattern is transferred to the substrate 11. For this reason, the amount of exposure is controlled upon measuring the amount of exposure at a position which is at a conjugate point with the substrate 11 and offset from the optical path of the exposing light.
Use is made of a half-mirror 13, which has a very low reflectivity, inserted into the optical path of the exposing light in order to produce a position which is at a conjugate point with respect to the substrate 11 and offset from the optical path of the exposing light. That is, the half-mirror 13 produces a position which is at a point conjugate with the substrate 11 and offset from the optical path of the exposing light at the position of an exposure sensor 14. The sensor 14 is placed directly in front of the point conjugate with the substrate 11 and at an inclination relative to the optical axis of the exposing light for the purpose of measuring the amount of exposure from the light diverted to it by the half-mirror 13.
The exposure sensor 14 is so adapted as to be capable of measuring an amount of exposure that corresponds to the amount of exposure exactly at the center of the illuminated area, namely at the position of the substrate 11 on the optical axis. Before the substrate 11 is exposed, the exposure sensor 15 mounted on the stage 12 is moved to the center of the illuminated zone, trial exposure is carried out and the relationship between the amount of exposure at the position measured by the exposure sensor 14 and the amount of exposure on the substrate 11 is found, thereby making it possible to estimate the amount of exposure on the substrate 11 from the output of the exposure sensor 14.
A controller 16 is for controlling the amount of exposure. On the basis of the output of the exposure sensor 14, and in accordance with a predetermined control program, the controller 16 controls the amount of exposure by controlling the opening and closing of a shutter 17, the transmittance of beam attenuating means 18 the transmittance of which is variable, and the input to the light source 1.
Assume that the half-mirror is used in the opticals unit. As may readily be deduced from the laws of opticals, a difference in reflectivity ascribed to the state of polarization of a ray of light increases with an increase in the angle formed by the ray of light and the perpendicular to the half-mirror. As a consequence, the intensity of light in back of the mirror differs depending upon the state of polarization of the light. This influences the degree of illuminance unevenness in the illuminated area. In order to illuminate the illuminated area uniformly, therefore, it is required that the half-mirror be disposed so as to perpendicularly intersect the optical axis to the greatest extent possible.
In accordance with the prior art of FIG. 2, however, a ray of light reflected by the half-mirror 13 reaches the exposure sensor 14 directly. Consequently, as shown in FIG. 2, the half-mirror 13 must disposed in such a manner that a ray of light reflected from the half-mirror 13 to the exposure sensor 14 will not be obstructed by the lens 5 immediately in front of the half-mirror 13.
FIG. 4 illustrates light rays which arrive at the lens 5 immediately in front of the half-mirror 13 and at the illuminated area along the optical axis in the vicinity of the half-mirror 13 in the example of the prior art shown in FIG. 2. In order to arrange it so that the light reflected from the half-mirror 13 to the exposure sensor 14 will not be obstructed by the lens immediately in front of the half-mirror 13, the relation B less than Atanxcex8 should hold, where A represents the distance from the lens 5 to the half-mirror 13, B the radius of the lens 5 and xcex8 the angle defined by the perpendicular to the half-mirror 13 and the optical axis. Making the half-mirror 13 nearly perpendicular to the optical axis in order to reduce the difference in reflectivity due to the state of polarization corresponds to reducing the angle xcex8 formed by the reflected light ray and optical axis. In the example of the prior art, therefore, the distance A between the half-mirror 13 and the lens 5 is great.
In other words, in the example of the prior art, if it is attempted to insert the half-mirror 13 into the optical path so as to diminish the difference in intensity of light between one state of polarization and another in back of the half-mirror 13, the distance between the half-mirror 13 and the lens 5 becomes too large, thereby making necessary a large amount of space in order to be able to control the amount of exposure of substrate 11. However, as a result of the improved performance and capabilities sought for projection exposure apparatus, the opticals unit employed in such apparatus has become extremely complicated and there is a tendency for such apparatus to be of ever increasing size.
Accordingly, in order to reduce the size of a projection exposure apparatus even marginally, there is strong demand to reduce the space necessary for controlling the amount of exposure.
The present invention has been proposed to solve the problems of the prior art and its object is to provide an exposure control method, exposure apparatus and device manufacturing method in which it is possible to reduce the space necessary for measuring the amount of exposure for the purpose of controlling the same.
According to the present invention, the foregoing object is attained by providing a method of controlling amount of exposure in which when a pattern on a reticle is illuminated by illuminating light from a light source so as to be projected onto a substrate to expose the same, the illuminating light is diverted by a half-mirror, the amount of exposure is measured on the optical path of the diverted illuminating light and the amount of exposure of the substrate is controlled based upon result of such measurement, wherein the position of the half-mirror is set in such a manner that the optical path of the diverted illuminating light passes through part of an optical element situated on a side of the half-mirror that faces the light source, and the amount of exposure is measured on the optical path of the diverted illuminating light following its passage through the optical element.
In accordance with a preferred embodiment of the present invention, the method includes a step of performing control of the amount of exposure of the substrate while taking into account a relationship, which has been found in advance, between a measured value of amount of exposure on the optical path of the diverted illuminating light and the amount of exposure of the substrate.
In accordance with a preferred embodiment of the present invention, the method includes steps of finding a relationship between an amount of exposure on the optical path of the diverted illuminating light and amount of exposure of the substrate at a predetermined position on the substrate before the substrate is exposed, and performing control of the amount of exposure of the substrate while taking into account this relationship and unevenness in illuminance in an illuminated area on the substrate when the substrate is exposed.
Further, according to the present invention, the foregoing object is attained by providing an exposure apparatus comprising: projection exposure means for illuminating a pattern on a reticle by illuminating light from a light source so as to project the pattern onto a substrate to expose the same; exposure measurement means for measuring amount of exposure on an optical path to which the illuminating light has been diverted by a half-mirror; and exposure control means for controlling amount of exposure of the substrate based upon result of such measurement, wherein the position of the half-mirror is set in such a manner that the optical path of the diverted illuminating light passes through part of an optical element situated on a side of the half-mirror that faces the light source, and the exposure measurement means measures the amount of exposure on the optical path of the diverted illuminating light following its passage through the optical element; and the exposure control means finds a relationship between an amount of exposure on the optical path of the diverted illuminating light and amount of exposure of the substrate at a predetermined position on the substrate before the substrate is exposed, and performs control of the amount of exposure of the substrate while taking into account this relationship and unevenness in illuminance in an illuminated area on the substrate when the substrate is exposed.
In accordance with a preferred embodiment of the present invention, the projection exposure means is scanning-type projection exposure means which, while part of the reticle pattern is being projected onto the substrate, scans the reticle and the substrate synchronously to thereby scanningly project the reticle pattern onto the substrate.
Further, according to the present invention, the foregoing object is attained by a device manufacturing method for manufacturing a device by illuminating a pattern on a reticle by illuminating light so as to project the pattern onto a substrate to expose the same, wherein when exposure by projection of the illuminating light is performed, the illuminating light is diverted by a half-mirror, the amount of exposure is measured on the optical path of the diverted illuminating light and the amount of exposure of the substrate is controlled based upon result of measurement, the method comprising steps of setting the position of the half-mirror in such a manner that the optical path of the diverted illuminating light passes through part of an optical element situated on a side of the half-mirror that faces the light source and measuring the amount of exposure on the optical path of the diverted illuminating light following its passage through the optical element; and finding a relationship between an amount of exposure on the optical path of the diverted illuminating light and amount of exposure of the substrate at a predetermined position on the substrate before the substrate is exposed, and performing control of the amount of exposure of the substrate while taking into account this relationship and unevenness in illuminance in an illuminated area on the substrate when the substrate is exposed.
In the prior art, the angle of the half-mirror and the distance between the half-mirror and the lens are designed in such a manner that the optical path of the light diverted by the half-mirror will not strike the lens located immediately in front of the half-mirror, and the amount of exposure is measured upon guiding the light from the half-mirror directly to an exposure sensor. As a consequence, a large space is required to measure the amount of exposure. In accordance with the present invention, however, the amount of exposure is measured by guiding the optical path of the light, which has been diverted by the half-mirror, to the exposure sensor after the diverted light has passed through part of an optical element such as a lens situated on the light-source side of the half-mirror. As a result, less space is needed to measure the amount of exposure.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.